Mechanical plating in barrels on a commercial scale started in 1954 and, over the years, has grown steadily until today it is a sizeable industry. The plating process has remained essentially stable--it still uses coppering of the work, uses glass beads as impact media, uses Tin, either as metal powder or as Tin Salt. The chemistry has changed little over the years nd generally consists of such chemicals as polyexyethylene amines, polyoxyethylene amids, materials such as carbowax and the like. The flux started in 1954 with Citric Acid which was used for years. When this became too expensive, Clayton introduced Hydroxyacetic Acid (glycolic acid), and when this became too expensive he was the first to introduce Sulphuric Acid which is today almost always used. With the advent of Sulphuric, most suppliers switched to tilting coned open-ended barrels. The barrel operates in a tilted position that causes the Zinc dust to migrate towards the back of the barrel leading to uneven distribution of coating thickness. Another reason for using open-ended barrels is that it permits the use of strong solutions of Sulphuric Acid, strong enough to dissolve unused Zinc from the previous run and to clean the glass impact media. To solve these problems of migration of Zinc uneven coatings, the Zinc and chemicals are added repetitively every few minutes throughout the plating run. This is very labor-intensive as a workman has to stay almost constantly beside the barrel when additions of Zinc dust and chemicals are made every three to five minutes. After the strong acid cleaning in the tilting barrel, the coppering salts are added directly and the coppering proceeds until all the copper is plated on the work. The Tinning steps then follows. The Tin Salts are very expensive, approximately fifteen dollars a pound. The Copper is a toxic metal. Its bright red colors uncoated areas very plainly, and if the coated piece has Copper showing, the addition of the Tin Salts plates over the Copper and so disguises the fact that the Zinc coat is not completely continuous. This system does have the advantage that all of the operations of pickling the work, cleaning the glass beads, coppering, tinning, zinc plating and rinsing all are done in the same barrel. The disadvantage is that the barrel is tied up on all the surface preparation steps cutting down on production and increasing labor costs.
For decades the system was used for plating thin Zinc coatings in the electroplate range of 0.0002 inches to 0.00035 inches, i.e., two-tenths of a mil to three and a half tenths of a mil. With saturation of this market, attention was directed at Zinc plating of large heavy objects with substantial Zinc coatings in the galvanizing thickness range and directly competing with hot-dip galvanizing. Coatings on articles like large bolts had a thickness of 2.1 mil.
For high Zinc efficiency and rapid plating, the mass of glass media needs to be free-flowing and very permeable so the Zinc can move through the media with minimum impedance. When plating heaving work such as large bolts and pole line hardware, their mass and inertia is such that they tend to sink through the media instead of rising nearly to the top of the barrel. Their mass is such that they lack sufficient turn over, or random movement. The impact media needs more lift, and the ability to reorient the work as it tumbles.
To maintain a permeable mass of glass and provide sufficient lift, large glass balls about five millimeters are added. The trouble with the large glass balls is that they are very difficult to pump--so much so that they are another major reason for using the open-ended tilt barrel.
The forced use of the open-ended cone-shaped tilting barrel cuts down on production so much that in a large tilting barrel the actual work may well be reduced by four to six hundred pounds of work over what it would be in a horizontal barrel and the barrel is also tied up during the pickling step. The larger the barrel load the greater the chance of chipping on heavy objects needing a heavy Zinc coating. For reasons such as this and those related above, a maximum load of 2 lbs per object and 12" in length is specified in the 1987 Metal Finishing Handbook in the article on Mechanical Barrel Plating, p. 350. In sharp contrast, with the use of rubber impact, large pieces weighing 4-10 lbs can be plated in bulk without chipping.
These limitations restrict marketability since a large part of the market is lost to competing processes such as hot-dip galvanizing.
The chemistry of the barrel plating appears to have changed little--usually consists of a Sulphuric Acid flux, a mixture of hydrocarbon filming agents or surfactants, and a defoaming agent such as an anti-foaming silicone.
Foam is most undesirable in the barrel because it carries a major portion of the glass beads out of the rotating mass, and may remove virtually all of the finer portions of impact media that are necessary to reach into recessed inaccessible areas such as the thread roots of the workpiece.
From the very early days of commercial mechanical plating in barrels, the impact material has been glass beads or glass balls or a combination of both. They are still universally used today for this purpose.
The glass beads have serious disadvantages:
1. They limit the size of the articles that can be plated. Heavy articles in excess of 1 or 2 pounds break up the glass, especially the finer fractions.
2. Even with the use of larger glass balls, the glass mixture is not very permeable. The Zinc dust has to migrate to the working being plated. Additions of large quantities of Zinc dust seriously hinder the migration of the Zinc powder. The larger the amount of Zinc added, the worse the problem. This has lead to the use of frequent but small additions of Zinc powder.
With constant use there is a breakdown of the finer sizes of glass. These further complicate the migration problem, especially as the broken glass mixed with Zinc powder makes the mixture more impermeable.
It is essential that the glass be cleaned and free of contaminating films. The glass impact media must be thoroughly washed after each run and all unused Zinc removed. This is essential for successful plating in the mechanical plating process.
The handling of a large volume of glass beads is timeconsuming and costly especially as usually 2 pounds of glass impact media are used for each pound of work processed. The space occupied by the large mass of glass impact media cuts down the production per barrel load and the removal and return of glass for each plating cycle further reduces production because of the time lost.
The above is believed to be an accurate recital of the present Mechanical Plating in Barrels and especially the mechanical plating of heavy objects. This recital of the prior art is necessary to understand the improvements of this invention.